1. Field of the Invention
The present invention relates to a surface acoustic wave filter used for a band filter, and more particularly to a ladder surface acoustic wave filter including multiple single-port surface acoustic wave resonators.
2. Description of the Related Art
In recent years, technology in the field of reduction of the size and weight of communication devices such as cellular phones has made remarkable progress. As filters used for such communication devices, surface acoustic wave filters are being used more often due to the small size and weight thereof while still being able to meet the demand for a high operation frequency band.
A ladder bandpass filter including single-port surface acoustic wave resonators that are connected with alternating serial arms and parallel arms is disclosed in Japanese Unexamined Patent Application Publication No. 5-183380. With this ladder filter, first single-port surface acoustic wave resonators are serially connected, and second single-port surface acoustic wave resonators are connected in parallel, wherein the antiresonant frequency of the parallel resonators is substantially matched with the resonant frequency of the serial resonators. With this configuration, a filter having outstanding properties, such as low loss and a wide band width, is obtained.
Furthermore, a filter with a high frequency band is obtained by adding serial inductors to the serial resonators or the parallel resonators.
However, in general, not only a wide frequency band width but also an excellent reflection property and acute and deep attenuation property near the pass band are required. In particular, increased attenuation is required in the diplex band, and the closer the pass band is to the diplex band, the more the attenuation property must be increased. For example, when the Rx frequency band is close to the pass band on the high-frequency side, such as in a PCS-Tx RF filter, the attenuation properties of the filter near the pass band width on the high-frequency side must be increased.
Furthermore, a surface acoustic wave filter is described in Japanese Unexamined Patent Application Publication No. 7-154201, wherein vertical connection steps of the surface acoustic resonators are increased in a chip having a conventional chip size. In this case, a problem is described wherein a direct wave (bridging capacity) from the input electrode pad or output electrode pad to the signal line, or from a serial arm signal line to a parallel arm signal line, prevents sufficient attenuation. In order to solve the aforementioned problem, four methods are disclosed in the aforementioned Publication. The four methods are 1) a method of providing a grounded electrode pattern between the input electrode pad or the output electrode pad and the signal line, 2) a method wherein, in a surface acoustic wave filter in which at least two surface acoustic wave resonators are serially connected with serial arms, grounded electrode patterns are provided between the surface acoustic wave resonators connected to the serial arms, 3) a method of providing a grounded electrode pattern between the serial arm signal line and the parallel arm signal line, or between the parallel arm signal line and the parallel arm signal line adjacent thereto, and 4) a method wherein, in a surface acoustic wave filter in which serial-arm surface acoustic wave resonators and parallel-arm surface acoustic wave resonators are disposed at positions adjacent to one another, in parallel with the surface acoustic wave transmission direction, a grounded electrode pattern is provided between the serial-arm surface acoustic wave resonator and the parallel-arm surface acoustic wave resonator.
However, with the methods disclosed in Japanese Unexamined Patent Application Publication No. 7-154201, the direct wave is only reduced between the input electrode pad or the output electrode pad and the signal line, between the serial resonators which are serially connected, between the serial-arm signal line and the parallel-arm signal line, between the parallel-arm signal line and another one adjacent thereto, and between the serial resonator and the parallel resonator disposed at positions adjacent to one another in parallel with the surface acoustic wave transmission direction.
On the other hand, the demand for reduction in size of the surface acoustic filter is increasing. In order to reduce the size of the surface acoustic filter, the size of the piezoelectric substrate (chip size) used for the surface acoustic wave filter must be reduced. Such reduction in the chip size causes additional adverse effects. For example, direct waves (bridging capacity) actually arrive. Such direct waves include not only a wave which is directly transmitted between electrodes, but also include a wave which is transmitted by making a detour around the electrode. When the surface acoustic wave filter has a large chip size, the direct wave does not cause a substantial problem. However, when the surface acoustic wave filter has a small chip size, the distance between the electrodes provided on the chip is reduced, and consequently, the influence of the direct wave greatly increases, which leads to a substantial problem. With the surface acoustic wave filter, a large direct wave arriving from the input electrode to the output electrode causes significant effects upon the properties of the surface acoustic wave filter. With the ladder surface acoustic wave filter, in particular, there are significant effects on attenuation near the pass band on the high-frequency side. The arrival of the direct wave is caused by the distance between the electrodes provided on the chip being reduced in the same manner as with the arrangement disclosed in Japanese Unexamined Patent Application Publication No. 7-154201, wherein vertical connection steps are increased in a chip having a conventional chip size.
Here, as an example, FIG. 15 illustrates a schematic configuration of a ladder surface acoustic filter 120 having a conventional chip size (1.05 mm by 1.55 mm). FIG. 16 is a circuit diagram which illustrates the surface acoustic wave filter.
As shown in FIG. 15, the conventional surface acoustic wave filter 120 includes an input electrode pad 113a, an output electrode pad 113b, and grounded electrode pads 114a and 114b, provided on a piezoelectric substrate 110, and furthermore, includes serial resonators 111a through 111c and parallel resonators 112a and 112b arranged in a ladder configuration. The serial resonator 111a is connected to the input electrode pad 113a, and the serial resonator 111c is connected to the output electrode pad 113b. Furthermore, the parallel resonator 112a is connected to the grounded electrode pad 114a, and the parallel resonator 112b is connected to the grounded electrode pad 114b. FIG. 17 illustrates the filter property of the surface acoustic wave filter 120.
On the other hand, FIG. 18 illustrates a schematic configuration of a ladder surface acoustic wave filter 130 having a reduced chip size (0.90 mm by 1.05 mm) with the same configuration as the surface acoustic wave filter 120. FIG. 19 illustrates the filter property of the surface acoustic wave filter 130.
As shown in FIGS. 17 and 19, attenuation near the pass band on the high-frequency side is reduced. This is because the chip size is reduced and the electrode layout is changed, and the distance between the input electrode pad 113a and the output electrode pad 113b is reduced.